How to update ngModel from directive?

Question

I've created a directive to limit the length of input field type=number.

// Input

<input min="1" appLimitTo [limit]="5" type="number" name="property" [(ngModel)]="property">

// Directive

import {Directive, HostListener, Input} from '@angular/core';

@Directive({
  selector: '[appLimitTo]',
})
export class LimitToDirective {

    @Input() limit: number;
    @Input() ngModel: any;

    @HostListener('input', ['$event'])
    onInput(e) {
        if (e.target.value.length >= +this.limit) {
            e.target.value = (e.target.value).toString().slice(0, this.limit - 1);
            e.preventDefault();
        }
    }
}

It works fine if we enter the value through the keyboard. But the problem arises when if I copy & paste 12345678913465789 this number, this line e.target.value = (e.target.value).toString().slice(0, this.limit - 1); shorten it to the limit, but the ngModel still contains 12345678913465789 value. How to update this ngModel value?

Please help.

PS - What should I add in my directive to meet the requirement?

Answer 1

You can inject NgControl into your own directive. You can then listen to the control valueChanges event.

limit-to.directive.ts

import {Directive, HostListener, Input, OnInit, OnDestroy} from '@angular/core';
import {NgControl} from '@angular/forms';
import {map} from 'rxjs/operators';
import {Subscription} from 'rxjs/Subscription';

@Directive({
  selector: '[appLimitTo]',
})
export class LimitToDirective implements OnInit, OnDestroy {
    @Input('appLimitTo') limit: number;

    private subscription: Subscription;

    constructor(private ngControl: NgControl) {}

    ngOnInit() {
      const ctrl = this.ngControl.control;

      this.subscription = ctrl.valueChanges
        .pipe(map(v => (v || '').toString().slice(0, this.limit)))
        .subscribe(v => ctrl.setValue(v, { emitEvent: false }));
    }

    ngOnDestroy() {
      this.subscription.unsubscribe();
    }
}

usage:

<input ngModel appLimitTo="3" type="number" />

Live demo